The ATM Adaptation Layer type 2 (AAL-2) standard, as defined in the ITU-T I.363.2 recommendation, provides for bandwidth efficient transmission of low-rate, short, and variable length packets in delay sensitive applications. AAL-2 enables multiplexing of different user information streams into a single logical connection (ATM-VCC). These user information streams may include voice, compressed voice and data, real-time user information and non-real-time user information, which have different quality of service demands, such as transfer delay and latency.
The Service Specific Segmentation and Reassembly Sublayer (SSSAR) segments (and reassembles) complete frames of information from user information streams into Common Part Sublayer (CPS) service data units (CPS-SDU) that form the payload of CPS packets. CPS packets originating from different applications have different channel identifiers (CID) and may be multiplexed into the same CPS protocol data unit (CPS-PDU) for transmission in a single ATM cell.
The AAL-2 standard sets the maximum length of a CPS-SDU to be either 45 octets (the default) or 64 octets. However, the standard neither specifies the actual length of the segments nor does it specify how to determine this length. Typically, the segment length is set as a general parameter that is predefined during the establishment of a channel (ATM-VCC). Using a fixed segment length per logical ATM connection is a compromise that balances the traffic stream (user information), the current and average connection bandwidth utilization and the quality of service demands of the voice and high-priority data. It may lead to frequent use of the “split” and “part” state variables, which in turn may result in difficulty meeting jitter and delay demands of a voice application. It may also lead to inefficient utilization of the bandwidth, and, in some cases, may cause extra processing power to be spent segmenting and reassembling unnecessary CPS-SDUs.
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